1. Field of the Invention
The present invention relates generally to inkjet printing. More specifically, the present invention relates to inkjet printing onto an intermediate transfer medium coated with a liquid coating solution.
2. Description of the Related Art
Early inkjet printers had several distinct advantages over laser printers. They could print in color and had size and cost advantages. However, in order for the next generation of inkjet printers to compete with laser printers in an office environment, it will be necessary to increase the printing speed and print quality. As a goal, a printing speed of 20 pages per minute is desired. For print quality, a black print density of 1.4 as measured on an optical densitometer and a resolution of at least 600 dots per inch are desired.
To achieve these goals and successfully compete with laser printers in the business printer market, it has been proposed to print from an inkjet head onto an intermediate transfer surface (e.g. a drum) and then transfer the image onto a final medium (e.g. paper). Prior attempts at this intermediate transfer type printing have resulted in poor transfer efficiency, that is, the image printed onto the intermediate transfer surface did not substantially completely transfer to the final medium. Some of the material deposited on the intermediate transfer surface remained after transfer. Prior attempts at using an intermediate transfer medium with inkjet printing have also resulted in poor optical density.
Accordingly, it is an object of the present invention to increase the speed of inkjet printing.
It is a further object of the present invention to increase the optical density of images printed by inkjet printing.
It is a still further object of the present invention to enable inkjet printing using an intermediate transfer medium in which the transfer efficiency to the final medium is increased.
These and other objects are accomplished by a layered intermediate used in inkjet printing. The layered intermediate contains a release coat formed on a transfer medium in an area where an image has not been printed. The release coat is formed of a water soluble polymer or a water soluble copolymer and at least one hydroxyl containing solvent. Polyvinyl pyrrolidone (PVP) and polyvinyl pyrrolidone copolymer may serve as the water soluble polymer and water soluble copolymer, respectively. Glycol solvents or diol solvents may be used as the hydroxyl containing solvent. A viscous coating formed of PVP or PVP copolymer, for example, from the release coat is formed on the transfer medium in an area where an image has been printed. A release layer formed from components in a coating solution and components in an ink is formed on the viscous coating.
A first ink layer of flocculated ink is formed on the release layer. A second ink layer is formed on the first ink layer, of ink that is substantially not flocculated. The first and second ink layers contain about 0.01 to about 15 wt. % of a wetting agent selected from the group consisting of 1,2 alkyldiols having 4-10 carbon atoms and diether alcohols having 6-14 carbon atoms.